Direct detection optical signal receivers have been used in optical networks for recreating data modulated on an optical signal and transmitted on an optical path to the receiver. In general, a direct detection receiver may demodulate the data by detecting the phase and/or amplitude of received symbols representative of the transmitted data. The detected phase and/or amplitude may be provided to hard and/or soft decision detector configurations and forward error correction (FEC) circuits to generate an output bit stream that is representative of the data modulated on the signal at the transmitter.
One challenge in wavelength-division multiplexed (WDM) optical networks including direct detection receivers is that light propagating within an optical fiber may undergo chromatic dispersion, i.e. different wavelengths of the light may travel at different group velocities leading to varying wavelength-dependent delays in transmission. The chromatic dispersion imparted by an optical fiber causes the transmitted pulses to spread and overlap. To reliably detect the transmitted data, the chromatic dispersion should be removed before the receiver. Removing the dispersion imparted by the transmission line may, however, be impractical when the dispersion is larger than a few 1000's of ps/nm.
To address this issue, known systems have incorporated dispersion management techniques to reduce the dispersion at the receiver to practical levels. One known dispersion management technique involves dispersion mapping where optical fiber types are selected and arranged to manage the dispersion in the transmission segments of an optical communication system. One example of a dispersion mapped transmission segment mixes spans of non-zero dispersion-shifted fiber (NZDSF) or spans of dispersion flattened fiber (DFF) having a non-zero dispersion with spans of dispersion compensation fiber (DCF) to realize periodic dispersion compensation over the length of the optical transmission segment. The length of each period in such periodic dispersion maps may be in the range of about 500 km per period.
FIG. 1, for example, includes a plot 10 of accumulated dispersion (ps/nm) vs. distance associated with such a periodic dispersion map using DFF fibers. As shown, dispersion may be near zero at the receiver, i.e. at about 8500 km, and non-zero but small (e.g. a maximum accumulated dispersion of about 1500 ps/nm) along the system. For conventional systems using direct detection, such dispersion mapping techniques have been useful in maintaining a low end-to-end path average dispersion and suppressing fiber nonlinearities.
It has been recognized however, that coherent detection receivers may provide advantages over direct detection receivers. In general, a coherent receiver utilizes coherent detection, e.g. homodyne or heterodyne detection, to detect modulated optical signals. The term “coherent” when used herein in relation to a receiver refers to a receiver including a local oscillator (LO) for demodulating the received signal. Digital signal processing (DSP) may be implemented in such systems for processing the received signals to provide demodulated data. Digital signal processing of the received signal provides speed and flexibility, and may be used to perform a variety of functions. DSP can remove large amounts of chromatic dispersion, and can perform other functions such as correction of intersymbol interference and polarization dispersion. Thus, unlike direct detection receivers, coherent detection receivers do not require dispersion management. Unfortunately, systems built to support both direct detection and coherent detection receiver configurations must be dispersion managed to allow use of practical direct detection receiver configurations.